AFAIK tdlib is currently under "GPLv3 or later". I'd suggest to switch to a slightly more permissive license: "GPLv2 or later".
In my opinion, both GPLv2 and GPLv3 are sufficient to protect the ess…
Until the project is officially placed under a libre license, it is proprietary.
See https://freedomdefined.org for the definition of libre works and some libre licenses.